Syntactic foams and their preparation

ABSTRACT

A syntactic foam is produced by a controlled curing of a polymer which is a homopolymer of butadiene or a copolymer of butadiene and styrene or the like, at least 40% of which polymer is butadiene and at least 80% of the repeating butadiene units are of the 1,2-configuration. Instead of styrene, a methyl or ethyl derivative may be used. The syntactic foam includes minute hollow spheres which give strength to the foam product and the syntactic foam product has a very low density.

United States Patent [1 1 Fritschel Dec. 24, 1974 [54] SYNTACTIC FOAMS AND THEIR 3,594,361 7 1971 Parris et al 260/94.7 A PREPARATION 3,635,933 l/l972 Schaffhauser et a]. 260/94.7 A 3,635,934 111972 Schaffhauser et a]. 260/94] A [75] Inventor: Lar y Ed Frltschel, y ga 3,786,087 1/1974 Conard et al. 260/94.7 A

Falls, Ohio [73] Assignee: The Firestone Tire & Rubber Primary Examiner-Mrt0n Foelak Company, Akron, Ohio [22] Filed: Oct. 16, 1973 [57] ABSTRACT [21] Appl' 406793 A syntactic foam is produced by a controlled curing of a polymer which is a homopolymer of butadiene or a 6 60/25 HA, 260/2-5 H copolymer of butadiene and styrene or the like, at 2 /4 260/66, 260/851, least of which polymer is butadiene and at least 260/943 260/876 /879, 260/88 R, of the repeating butadiene units are of the 1,2- 260/887, 260/889, 260/893 configuration. Instead of styrene, a methyl or ethyl del CL Cosd C03d C 1/ rivative may be used. The syntactic foam includes Field of Search B minute hollow spheres which give strength to the foam product and the syntactic foam product has a very low [56] References Cited density.

UNITED STATES PATENTS 3,384,629 5/1968 Strobel 260/94.7 A 9 4 Drawmg F'gures 1 SYNTACTIC FOAMS AND THEIR PREPARATION FIELD OF THE INVENTION Syntatic foams are known in the art. The ASTM definition of a syntactic foam is material consisting of hollow sphere fillers in a resin matrix. The foams are used in deep-submergence vehicles, instrument packaging and electronic gear, cable buoys, floatation collars for deep-water drilling operations, etc. By using the curing system disclosed and claimed herein, improved syntactic foams are produced.

The polymeric material used in the production of syntactic foam when subjected to ordinary curing conditions is unusually exothermic and the heat build-up is so great in moderate or large-sized sections that unless it is controlled, the polymeric material becomes charred. This charring leads to severe strength reduction and an unusable product. By the controlled curing of this invention, charring is prevented. Syntactic foams of relatively low density can be produced.

BACKGROUND OF INVENTION Syntactic foams are known. They are discussed, for instance, in Heres Whats Happening in High Performance Syntactic Foams by Roger W. Johnson in Plastics Design and Processing for April, 1973 on pages 14-17.

The polymeric material of this invention is subjected to a two-stage cure. The first stage being a lowtemperature curing system utilizing methylethyl ketone (MEK) peroxide or other peroxides used in lowertemperature cures, cobalt naphthenate, iron naphthenate, and acetylacetone (pentanedione) or the like; the peroxide used in the second stage requiring a higher temperature for activation.

The curing of polyesters by peroxide combinations with cobalt naphthenate and iron naphthenate is known in the art and is disclosed in Accelerators for Peroxide Curing of Polyesters by Brinkman et al. on pages 167-172 of Modern Plastics for October, 1968.

Conard, Ser. No. 406,792 filed Oct. 16, 1973 discloses the use of cobalt and iron naphthenates, generally in larger amounts than disclosed herein in a twostage peroxide curing of the polymer to which this invention relates, together with pentanedione as an activator, but more particularly for the curing of thin films, the mixture of the two naphthenates being used to provide a tack-free product.

SUMMARY OF INVENTION Syntactic foams are formed of polymeric materials which include hollow spheres (microspheres), and if the temperature used in preparing the foam is sufficiently high to melt the spheres, the bubbles formed due to the presence of the spheres, are surrounded by melted remains of the spheres.

The polymeric material used in producing the syntactic foam of this invention is a resinous polymer of the class consisting of (1) homopolymers of butadiene or (2) copolymers of butadiene and styrene or a methyl or ethyl derivative of styrene such as alpha-methyl styrene or vinyl toluene, etc. At least 40% by weight of the polymeric material is butadiene, and at least 80% of the repeating butadiene units are of the 1,2-configuration.

Where foams of high strength are required, smaller microspheres are preferred, such as spheres measuring 20 to 300 or 400 or perhaps 500 microns in diameter and where strength is not critical, larger spheres may be used such as spheres measuring /2 inch up to 1 inch or more in diameter. Spheres of any size within these limits may be used. Thin-walled glass spheres are preferred for many operations where strength and thermal resistance are important, but spheres of other compositions including polyethylene, ABS, polymethyl methacrylate, and other plastics, both low-temperature and high-temperature-softening plastics, may be used. Even though the spheres are of such a composition as to be melted during the second stage of the cure, the effectiveness of the spheres is not materially reduced because during the first stage of the cure the structure of the foam is set. Therefore, the porosity of the syntactic foams used may be due to the presence of hollow beads, but the voids may be surrounded by the remains of the bead compositions. The foams must be substantially free from other bubbles of air or gas if maximum strength is desired.

The Cure The loading of the foam will depend upon the size of thespheres used. For the stronger foams smaller beads are preferred and maximum strength is obtained when the loading of the smaller beads is between and 100 parts per hundred weight of resin. Where strength is not so important, 30 to 50 parts of the larger beads may be used. Thus, the foam may include from 30 to 100 parts per 100 parts by weight of the resin. The weight of the larger spheres used in syntactic foams is progressively reduced as the size of the spheres increases.

The foam is produced by a two-stage cure. The first stage is carried out at about room temperature, any temperature between 50 and 150 F. being satisfactory and temperatures between 65 and F. being pre' ferred.

In the first stage, the cure is accomplished with a peroxide curing agent which decomposes at the temperature of the cure which is not over about 150 F., and. MEK peroxide is preferred. Other peroxides including bis(4-t-butyl cyclohexyl)-peroxydicarbonate, lauroyl peroxide, benzoyl peroxides, etc. may be used. The cure is effected without appreciable gas evolution because there is no substantial decomposition of the polymeric material during the cure. The amount ofthis peroxide employed will depend upon the size of the product produced. For larger articles about 0.2 to 0.5 parts of MEK peroxide per parts of the monomerextended resin will be used and for smaller articles, a somewhat larger amount will normally be employed, up to perhaps 5.0 parts per 100 parts of the monomerextended resin. Thus, from substantially 0.2 to 5.0 parts of the lower-temperaturue-curing; peroxide may be used.

The activators used in the controlled curing to which this invention relates are iron and cobalt naphthenates. The ratio of the amount of the colbalt naphthenate to the iron naphthenate is about 3-5/1. Five to 20 parts of cobalt naphthenate are used per 100 parts of the lower-temperature-curing peroxide used in the first stage of the cure.

The first stage of the cure utilizes pentanedione. (acetylacetone) or its equivalent as an accelerator. Substantially 10 to 30 parts of the accelerator, and preferably about 20 parts, are used per 100 parts of the low-temperature-curing peroxide. The pentanedione accelerates the decomposition of the metal naphthenates which in turn activate the peroxide. The pentanedlone is not active without the metal naphthenates.

The length of the induction period leading up to the first stage of the cure will depend upon the amount of ticles and the other type of cure in which additional heating is not necessary will be applied to large articles such as floatation collars, for instance. Such a collar is illustrated in the accompanying drawing.

the lower-temperature-curing peroxide, accelerator 5 In the drawing: and activator employed. Using a minimum amount of FIG. 1 is a foreshortened prospective view ofa floataaccelerator, the induction period may be as long as 5 tion collar for a drilling ring; days. Commercially, an induction period of about 3-5 FIG. 2 is an end view of the showing in FIG. 1; hours is preferred. At the end of the induction period, FIG. 3 is an enlarged foreshortened section on the the final temperature will rise to a temperature deterline 3-3 of FIG. 2; and mined by the size f the product being cu L F FIG. 4 is an enlarged detail of a section through the smaller products the temperature will not reach the ac- Collartivation temperature of the second stage peroxide. For The P p 1 is a p p to be driven l h bottom of larger products, the heat build-up due to the first stage a h Of e h y be 2 e dlarheler- The peroxide exotherm may be sufficient to activate the drllhhg'equlpmem 15 lowered "h The p e higher temperature peroxide. In the larger products, yh h foam each cover a p 0f the exterior P'P the levels of the curing agents are so adjusted that the whlch h larger than to Cover 301800 lz of the heat build-up is slow and gradual and although it may P P extenoh and uscuahy ahohl 60 to 90 h of the activate the second stage peroxide, it should not be suf- Sechohs 2 Covers 60 of the plpe surfacee P may ficient to adversely affect the polymer. By such control be 50 feet long and Such Sechoh may W Several of the curing which prevents temperature build-up to hundred e TWO seehohs Wh'eh encircle e plpe the point where charring or degradation of polymer oc are e m 1 whch ,Come he h the h curs, it is possible to cure products of substantial thicklg ggl l s e gtgf gs zgggh s2g z a hni l l gs t giili l t ii i z' eess -Such as products which elght mches or more places a substantial amount of water and thus the den- 1n thlckness. Unless the curing 1s controlled, the heat buildmp in such thick products, due to the peroxide slty of the assembly 15 mater ally less than that of the curing, is such that charring or degradation occurs to plpe alone The eeleehve of h Collar the extent that it reduces the strength of the product. h severe] 5.6mm of eoher whleh enclose e The second sta e of the cure is effected b a eroxide portion of the plpe are bonded together ee by the etrep g y p 4 the outwardly bent ends of wh1ch are held together 3 wh1ch 1s activated only at a higher temperature, eg y bolts and nuts 5 o o I 175 9 Unless the .hrst Stage 15 contlolled so Each section is composed of microspheres 6 disthat this hlgher temperature 1s not reached unt1l the deparsed in the plastic 7 sired state of a cure has been attained by the lower-temperature-curing peroxide, the highertemper- EXAMPLES athre'ehnhg perexde, h the Second Stage 'h In the examples, the iron and cobalt naphthenates are qhlekly generate shfheleht l to eeuee deeempesl used in solutions which contain 6% of the metal ions in non er h' of h poll/meme matenel' the respective materials. The MEK peroxide is used as e perexlde aehvated h seeehel stage of the a solution of 60% MEK peroxide in dimethylphthalate cure 1s a h1gh-temperature-cur1ng peroxlde such as t- 40 (Lupersol DDM) The extra Strength microspheres butyl .perhehzoater dcumyl heroxldhy Cumehe hydro (which are relatively large) to which the examples refer peroxlde, etc. If the second stage cure has not been acare 0C6] inorganic microspheres manufactured by til/filed y the lower'tempereture'curmg Perexlder Philadelphia Quartz Co. The glass microspheres are lhg y be effected y Phlelhg the matenal an oven smaller and are B30B microspheres manufactured by or otherwise heatmg 1t at a temperature necessary to Minnesota Mining Company. Larger beads may be activate the hlgh"temperature'ehrlhg Peroxide The used. Also, mixtures of small microspheres with large level of hlgh'lemPerawre-eurlhg Peroxide necessary to beads, omitting substantially all beads of intermedial achieve the desired final State of Cure y y from size, very low density products are obtained which have to l3arts P 100 Parts of mohomef-exlehded good strength. The polymeric material to which the inreslh- 5e vention relates and which is disclosed above is utilized The examples illustrate the two Separate Stages of the as monomer-extended resin. Other monomers reactive cure in which additional heat is applied in the second i h h polymer may b ili d h as di h stage as well as examples in which the control of the Zene, i ll l cyanurate, Styrene, m first stage of the cure provides sufficient generation of Th recipes for several products are given in Table I, heat to initiate or activate the second stage of the cure. d h nditi under whi h th d t w The examples in which the second stage of the cure is cured and their compressive strengths are given in initiated by further heating may be applied to small ar- Table II.

TABLE I Compounding Recipes EXAMPLE NO. 1 2 3 4 5 6 Monomer-Extended Resin* 80 80 80 80 80 Vinyl Toluene 20 "0 20 20 20 20 Sartomer 350** 3.4 3.4 3.4 2.0 2.0 2.0 Silane Z-6075*** 1.0 1.0 1.5 1.0 1.0 1.0 Cobalt Naphthenate .068 .04 .04 .04 .032 .04 Iron Naphthenate .015 .0l5 .0l5 .015 .012 .015

TABLE I -Continued Compounding Recipes EXAMPLE NO. 1 2 3 4 5 6 Extra Strength Mierospheres 20 20 Glass Microspheres 44.5 44.5 44.5 Lupersol DDM (60% MEK Peroxide) 2.0 1.0 .5 .5 .4 .5 Pentanedione .4 .2 .1 .l .08 .1 t-butyl perbenzoate .75 .75 .75 .75 .75 .75

65 parts of butadienestyrene (60/40) and parts of vinyl toluene. the copolymer having a dilute solution viscosity of 0.3. Trimethylolpropane trimethacrylate Vinyltriacetoxysilane TABLE I1 Curing Conditions and Properties EXAMPLE NO. 1 2 3 4 5 6 Sample Size 1 gallon 1 gallon 1 gallon 1 gallon 1 cu. ft. 1 cu. ft. 1st Stage Cure 16 hrs. 16 hours 16 hours 16 hours 120 hours 16 hours at 72 F. at 72 F. at 72 F. at 72 F. at 72 F. at 72 F. Peak Temperature 290 F. 240 F. 200 F. 150 F. 290 F. 290 F. Compressive Strength. psi 1588 2965 592 237 4183 5371 2nd Stage Cure 4 hours 4 hours 4 hours 6 hours None 24 hours at 250 F. at 250 F. at 250 F. at 250 F. at 250 F. Peak Temperature 250 F. 260 F. 300 F. 280 F. 250 F. Compressive Strength, psi 2729 4060 5158 6734 5234 Density lbs/cu. ft. 36.0 36.0 36.0 34.2 34.2 34.2

The peak temperatures refer to temperature within the material being cured. The peak temperatures obtained in Examples 1 and 2, during the first stage of the cure, were sufficient to activate the higher-temperature-curing peroxide used in the second stage of the cure. This resulted in continuation of the cure effected by the lower-temperature-curing peroxide. The products of the first four examples were small, measuring no more than substantially 8 inches in the largest dimension, and actually were small cylinders measuring 6 /2 inches in diameter and 7 inches in height. As is clear from the information given in Table 11, some of the higher-temperature-curing peroxide was decomposed and used up in the first stage of the cure. The final compressive strength was lower than in Examples 3 and 4. The peak temperatures reached in the first stage in Examples 3 and 4 did not activate the higher-temperature-curing peroxide which was active in the second stage of the cure. Examples l-4 were then heated in an oven at 250 F. for the indicated periods to effect the second stage of the cure. Comparing the data in the table which refers to the first four examples, it is evident that in the first two examples the higher-temperature-curing peroxide was somewhat activated in the first stage of the cure, whereas in the third and fourth cures, it was not activated during the first stage of the cures. By use of all of the high-temperature-curing peroxide in the second-stage cure in examples 3 and 4, high compressive strengths were obtained in the final products, much greater than those obtained in examples 1 and 2 in which some of the high-temperaturecuring peroxide had been activated during the first stage of the cure.

The larger samples of Examples 5 and 6 retained the heat generated during the cure in the first stage so that the high-temperature-curing peroxide was activated by the first-stage peroxide. This was not accomplished until the first stage of the cure had reached such a point that the activation of the second-stage peroxide did not cause an uncontrollable exotherm.

The density of the syntactic foams may be varied by using beads of different sizes and in different amounts. For a low density foam, it is necessary to use large beads and they may be used in larger and smaller amounts. Foams containing only small beads. i.e. microspheres less than about 200 microns in the largest dimension, the amount of the beads being to parts per 100 parts of the resin, have a density of be tween 36 and 32 pounds per cubic foot and a compres sion strength greater than 4,000 pounds up to and over 5000 pounds per square inch. As stronger microspheres are developed, foams with a density as low as 25 to 30 pounds per cubic foot will be produced, using the curing system disclosed herein. Example 6 is illustrative of the manufacture of such a foam. Foams of such properties, having a compression strength of over 5,000 pounds per square inch and measuring over 8 inches in the smallest dimension are believed to be new regardless of the polymer utilized.

l claim:

1. The process of producing a syntactic foam which comprises curing in two stages a monomer-extended resinous polymeric composition in which are dispersed substantially 30 to 100 parts of microspheres per 100 parts of the polymer, the microspheres measuring substantially 20 microns to 1 inch in diameter, the size and amount of the microspheres being selected to produce a commercial foam, the polymeric material being at least largely a polymer selected from the class consisting of (a) homopolymers of butadiene and (b) copolymers of styrene and methyl and ethyl derivatives of styrene, in which at least 40% by weight is butadiene and at least 80% of the repeating butadiene units being of the 1,2-configuration, 100 parts of said polymeric material being compounded with 0.2 to parts of a low-temperature-curing peroxide curing agent, 0.1 to 5 parts of a high-temperature-curing peroxide curing agent, a mixture of 3-5 parts of cobalt naphthenate to 1 part of iron naphthenate with 5 to 20 parts of cobalt naphthenate per 100 parts of the low-temperaturecuring peroxide, and to 30 parts of acetylacetone as a curing accelerator per 100 parts of the low-temperature-curing peroxide; in the first stage, curing at a temperature of 50 to 150 F. until the polymer sets, without substantial heat-degradation of the polymeric material, and thereafter, in the second stage, completing the cure at a temperature of 175 to 375F at which the higher-temperature-curing peroxide is effective.

2. The process of claim 1 in which the article of the polymeric composition is of such large dimensions that the heat retention is sufficient to initiate activation of the high-temperature-curing peroxide sufficiently to complete the cure without supplying additional heat.

3. The process of claim 1 in which the article of the polymeric composition is of such small dimensions that the loss of heat during the first stage of the cure is suffi cient to prevent an activation of the high-temperaturecuring peroxide, and heat is necessarly applied to complete the second stage of the curing with the high-temperature-curing peroxide.

4. The process of claim 1 in which methylethyl ketone peroxide is used in the first stage of the cure.

5. The process of claim 1 in which t-butyl perbenzoate peroxide is used in the second stage of the cure.

6. The process of claim 1 in which the microspheres measure substantially 20 to 500 microns in the largest dimension and to parts by weight of microspheres are used per 100 parts of the polymer.

7. The process of claim 1 in which the hollow beads measure 500 microns to 1 inch in diameter and 30 to 500 parts are used per 100 parts of the polymer.

8. A syntactic foam produced by the process of claim 1, which foam has a density of 25 to 36 pounds per cubic foot and a compression strength of over 4,000 pounds per square inch.

9. A syntactic foam produced by the process ofclaim l which foam has a density of 36 to 32 pounds per cubic foot, a compression strength of over 5,000 pounds per square inch and measures greater than 8 inches in the smallest dimension. 

1. THE PROCESS OF PRODUCING A SYNTACTIC FOAM WHICH COMPRISES CURING IN TWO STAGES A MONOMER-EXTENDED RESINOUS POLYMERIC COMPOSITION IN WHICH ARE DISPERSED SUBSTANTIALLY 30 TO 100 PARTS OF MICROSPHERES PER 100 PARTS OF THE POLYMER, THE MICROSPHERES MEASURING SUBSTANTIALLY 20 MICRONS TO 1 INCH IN DIAMETER, THE SIZE AND AMOUNT OF THE MICROSPHERES BEING SELECTED TO PRODUCE A COMMERICAL FOAM, THE POLYMERIC MATERIAL BEING AT LEAST LARGELY A POLYMER SELECTED FROM THE CLASS CONSISTING OF (A) HOMOPOLYMERS OF BUTADIENE AND (B) COPOLYMERS OF STYRENE AND METHYL AND ETHYL DERIVATIVES OF STYRENE, IN WHICH AT LEAST 40% BY WEIGHT IS BUTADIENE AND AT LEAST 80% OF THE REPEATING BUTADIENE UNITS BEING OF THE 1,2-CONFIGURATION, 100 PARTS OF SIAD POLYMERIC MATERIAL BEING COMPOUNDED WITH 0.2 TO 5 PARTS OF A LOW-TEMPERATURE-CURING PEROXIDE CURING AGENT, 0.1 TO 5 PARTS OF A HIGH-TEMPERATURE-CURING PEROXIDE CURING AGENT, A MIXTURE OF 3-5 PARTS OF COBALT NAPHTHENATE TO 1 PART OF IRON NAPHTHENATE WITH 5 TO 20 PARTS OF COBALT NAPHTHENATE PER 100 PARTS OF THE LOW-TEMPERATURE-CURING PEROXIDE, AND 10 TO 30 PARTS OF ACETYLACETONE AS A CURING ACCELERATOR PER 100 PARTS OF THE LOW-TEMPERATURE-CURING PEROXIDE; IN THE FIRST STAGE, CURING AT A TEMPERATURE OF 50* TO 150*F. UNTIL THE POLYMER SETS, WITHOUT SUBSTANTIAL HEAT-DEGRADATION OF THE POLYMERIC MATERIAL, AND THEREAFTER, IN THE SECOND STAGE, COMPLETING THE CURE AT A TEMPERATURE OF 175* TO 375*F AT WHICH THE HIGHER-TEMPERATURE-CURING PEROXIDE IS EFFECTIVE.
 2. The process of claim 1 in which the article of the polymeric composition is of such large dimensions that the heat retention is sufficient to initiate activation of the high-temperature-curing peroxide sufficiently to complete the cure without supplying additional heat.
 3. The process of claim 1 in which the article of the polymeric composition is of such small dimensions that the loss of heat during the first stage of the cure is sufficient to prevent an activation of the high-temperature-curing peroxide, and heat is necessarly applied to complete the second stage of the curing with the high-temperature-curing peroxide.
 4. The process of claim 1 in which methylethyl ketone peroxide is used in the first stage of the cure.
 5. The process of claim 1 in which t-butyl perbenzoate peroxide is used in the second stage of the cure.
 6. The process of claim 1 in which the microspheres measure substantially 20 to 500 microns in the largest dimension and 75 to 100 parts by weight of microspheres are used per 100 parts of the polymer.
 7. The process of claim 1 in which the hollow beads measure 500 microns to 1 inch in diameter and 30 to 500 parts are used per 100 parts of the polymer.
 8. A syntactic foam produced by the process of claim 1, which foam has a density of 25 to 36 pounds per cubic foot and a compression strength of over 4,000 pounds per square inch.
 9. A syntactic foam produced by the process of claim 1 which foam has a density of 36 to 32 pounds per cubic foot, a compression strength of over 5,000 pounds per square inch and measures greater than 8 inches in the smallest dimension. 